1. Field of the Invention
This invention relates to the production of high strength films, tapes, fibers and filaments from never-processed high molecular weight polyolefins. More specifically, it relates to the production of ultra high molecular weight (greater than 1.times.10.sup.6) polyolefin which can be compression molded below its melting point and thereafter drawn to draw ratios greater than 18:1 to produce high strength films, tapes, fibers and filaments.
2. Background
Polyolefin fibers of high strength and high modulus can only be prepared by additional processing of the virgin polymers. For example, polyolefin fibers of high strength and high modulus can be prepared by melt-spinning and subsequent tensile drawing. Wu and Black, Polym. Eng. Sci., 19, 1163 (1979), and U.S. Pat. No. 4,276,348 disclose the preparation of polyethylene filaments having a tensile strength of 19 g/den. Filaments of higher strength usually cannot be prepared by the method of melt-spinning. The use of higher molecular weight polymers (M.sub.w &gt;300,000) may provide higher strength filaments, but such polymers are difficult to process from the melt.
Polyolefin fibers having tensile strengths of more than 20 g/den are commonly produced only from solution. High molecular weight polymer (M.sub.w &gt;800,000) is dissolved in a suitable solvent to a concentration of 1-15%, by weight; the solution is then spun and cooled to form gel fibers which are either extracted, dried and drawn, or directly drawn to a draw ratio of 20 or more. Conventional draw procedures are used, such as disclosed in U.S. Pat. Nos. 4,344,908; 4,411,854; 4,422,993; 4,430,383; and 4,436,689 and in European Patent Application Publication No. 0 064 167. Such processes require the technologically difficult dissolution of high molecular weight polymer, and are frequently accompanied by mechanical degradation of the macromolecules, the latter having a negative effect on the final tenacity. The solution spinning route to high strength filaments also requires the evaporation of the spinning solvent, or extraction and drying, a tedious and costly operation.
Polyolefin films have been prepared and described by Chanzy et al., Kolloid-Z.u.Z. Polymer, 251, 563 (1973). Polyolefin films having an unusually high melting temperature and crystallinity have been described by Chanzy et al., Colloid & Polym. Sci., 252, 8-14 (1974). These publications do not disclose or suggest the hot-drawing of the films, and highly drawn films with high modulus and high tenacity are neither disclosed nor suggested. Chanzy et al., Polymer. 8, 567 (1967), discuss the morphological changes which occur during the stretching of a strip of virgin polyethylene, referred to as "nascent polyethylene". No highly oriented structures with high tenacity and high modulus are disclosed or suggested, and the strips can be drawn only to a draw ratio of about 2:1 at 25.degree. C. (room temperature) without breakage. Drawing at elevated temperatures is neither disclosed nor suggested.
Truss et al., Poly. Eng. Sci., 20(11), 747-755 (1980), disclose that ultra high molecular weight polyethylene [MW 1,000,000 to 5,000,000] can be fabricated from powder by cold compaction and sintering, provided polymer with a suitable size and morphology is used and the sintering is done at or above the melting point of the polymer. When sintering was conducted below the melting point of the polymer, the resultant samples had low density and tensile strength.
It is an object of this invention to provide an economical process for the preparation of polyolefin films, tapes, fibers and filaments which have a high modulus and high strength directly from the virgin polymer without the need for additional processing. Another object is to provide highly deformed films, tapes, fibers and filaments of polyolefins. Another object of this invention is to provide a process for the production of ultra high molecular weight polyolefin which can be compression molded below the melting point of the polymer. Still another object is to provide compression molded polyolefin which can be drawn to draw ratios greater than 15:1. A further object is to provide polyolefin films, tapes, fibers and filaments having tenacities greater than 15 g/den and initial moduli greater than 500 g/den. These and other objects will become apparent from the following description.